1. Field of the Invention
The present invention relates to a bubble-through-type ink jet printing apparatus and to a heat keeping control method for the apparatus.
2. Description of the Related Art
An ink jet printing apparatus, in which ink is ejected through an ejection outlet as minute droplets to print character information, such as letters, numbers and symbols, and pictorial information, such as figures and patterns, has excellent merits as a high-definition and high speed image printing means. In particular, a method using a bubble (air bubble) generated by an electro-thermal transducer (hereinafter referred to as a "heater"), i.e., a so-called thermal ink jet recording system (which is disclosed, for example, in Japanese Patent Publications No. 61-59911.about.59914), is characterized in that it easily allows a reduction in apparatus size and an increase in image density.
Further, the thermal ink jet recording system has the following features: by energizing the heater for ejecting ink droplets (hereinafter referred to as the "ejection heater"), heat energy is generated to thereby generate a bubble in the ink. The growth of the bubble thus generated is greatly influenced by the temperature of the ink around it. At the interface between the bubble and the ink, two processes are going on: the process in which gas-phase molecules in the bubble migrate into the ink and the process in which liquid-phase molecules in the ink migrate into the bubble. The temperature of the ink around the bubble influences the latter process. When the temperature of the ink is high, a large amount of molecules migrate into the bubble, with the result that the bubble grows to a relatively large extent. Conversely, when the temperature of the ink is low, the amount of molecules migrating from the ink into the bubble is relatively small, so that the size of the bubble is smaller as compared to that in the case in which the temperature of the ink is high. The size of the bubble reflects the amount of ink pushed out by it (hereinafter referred to as the "ejection amount"). Thus, in a thermal ink jet recording head, the ejection amount is greatly influenced by the temperature of the ink portion in the vicinity of the heater. When the ink temperature is high, the ejection amount is large, and when the ink temperature is low, the ejection amount is small.
Generally speaking, in a low-temperature environment, the ink used in ink jet printing undergoes an increase in viscosity (hereinafter referred to as "thickening"), so that the volume of the ink ejected from the printing head decreases or the ejection of the ink cannot be smoothly effected. Further, in the above-described thermal ink jet recording system, the temperature of the ink influences the growth of the bubble generated, and the volume of the bubble decreases, thereby decreasing the ejection amount or making it difficult for the ink to be smoothly ejected.
Further, when the ejection of ink is not effected, the volatile ingredient of the ink is evaporated, so that the thickening of the ink occurs to a particular degree, thereby making it difficult for the ink to be ejected in the normal fashion. As stated above, in a low-temperature environment, the ejection becomes more difficult; in extreme cases, the ejection becomes impossible.
In conventional printing apparatuses, the printing head is kept warm in a low-temperature environment before or during the printing operation to thereby cope with such defective ejection or the impossibility of ejection, thereby reducing the viscosity of the ink and adjusting the condition such that the bubble can be easily allowed to grow.
There are two principal methods of keeping the printing head warm: according to one method, the ink droplet ejection heater is driven to generate heat in the printing head. According to the other method, the printing head is equipped with a heater for keeping it warm (hereinafter referred to as the "heat keeping heater").
The conventional thermal ink jet recording heads and the conventional heat keeping methods have the following problem: when the ink is heated to be kept warm by using the ejection heater, the temperature of the ink portion in the vicinity of the heater becomes too high as compared to the temperature of the other ink portion. As a result, after the start of the ejection of ink droplets, the ejection amount is large while the ink portion at high temperature stays in the vicinity of the heater, but, when that ink portion has been ejected and an ink portion at a relatively low temperature is supplied, the ejection amount decreases, which means that the ejection amount is not stable.
When a heat keeping heater is used, the heat keeping heater is arranged at some distance from the ejection heater, and the ink is heated by the heat conducted from the heat keeping heater, so that there is no concern that a particular ink portion will be heated, thereby making it possible to avoid the above-mentioned problem. However, this method has a problem in that it involves an increase in cost with respect to the printing head or the apparatus since it requires the preparation of the heat keeping heater, the provision of the wiring for the heat keeping heater, etc.
Several control methods are available when performing printing while keeping the printing head and the ink at a temperature not lower than a certain temperature.
In one method, the printing head is kept warm before starting the printing (or during non-printing period) and no heat keeping is effected during printing. In this method, the temperature of the printing head is gradually lowered during printing when the printing duty is low, with the result that the ejection amount gradually decreases. This is not much of a problem when it is characters that are to be printed. However, in the case of the printing of color graphics or the like, the change in ejection amount will lead to an acute change in tinge, so that this is not permissible in a printing apparatus required to perform color development control.
According to a technique, the following measure is taken to cope with the change in ejection amount due to the temperature of the printing head: the signal to be applied to the ejection heater consists of a plurality of pulses, and, before the main pulse for actually ejecting ink droplets from the printing head is applied, a short pulse (pre-pulse) having such an energy level as will not cause a bubble to be generated in the ink is applied to heat the ink portion in the vicinity of the ejection heater to thereby control the ejection amount. However, there is a limit to the range in which this control is effective. When the printing head is driven at a high frequency, there is no time left for applying the pre-pulse before the application of the main pulse, which means the driving frequency for the printing head is limited.
To cope with the problem of the temperature of the printing head being lowered during low duty printing in a low-temperature environment, there is a technique available according to which an ejection heater which is not used for printing or the heat keeping heater is used even during printing to thereby keep the printing head warm. However, when the heater for heat keeping is driven simultaneously with the driving of the ejection heater, the consumption of power in the printing head during printing increases, so that it is necessary to install a power source device having a larger current capacity. A considerable increase in cost would be unavoidable if a power source device with a large current capacity were employed.
Further, apart from the power source device, the heat keeping control during printing may be effected independently of the driving signal for printing. In that case, it is necessary to provide a flexible cable for transmitting signals from the printing apparatus body to the printing head, and wiring on the chip incorporating the heater. Further, also when a driving signal for heat keeping is prepared by using a gate array provided on the chip to effect heat keeping during printing by using an ejection heater not being used for printing, it is necessary to provide wiring for that purpose on the chip, so that the chip area increases. For example, to effect heat keeping control from the printing apparatus body, it is necessary to provide a wire in a flexible cable for the transmission, resulting in an increase in cost. Further, when a heater and the requisite wiring are prepared on a silicon wafer by semiconductor process, the number of chips that can be prepared on one wafer is small when the area of the chip to incorporate the heater, etc. is large. Further, the proportion of the number of chips defectively produced due to dust, etc. increases, resulting in a reduction in production yield.